LONDON, Oct 9 (Reuters) - Scientists have found two distinct genetic "signatures" for prostate cancer that may help doctors predict which patients have aggressive tumours, and designed experimental blood tests to read those genetic signs like barcodes.

The teams, whose work was published on Tuesday in the Lancet Oncology journal, believe tests developed from the signatures could eventually be used to tell which patients need immediate treatment.

"Prostate cancer is a very diverse disease - some people live with it for years without symptoms but for others it can be aggressive and life-threatening," said Johann de Bono, who led a study at Britain's Institute of Cancer Research. "So it's vital we develop reliable tests to tell the different types apart."

Researchers in Britain and the United States found that by reading the patterns of genes switched on and off in blood cells, they could accurately detect which advanced prostate cancer patients had the worst survival rates.

Prostate cancer is the second most common cancer in men after lung cancer. There were 899,000 new cases diagnosed worldwide in 2008, the last year for which there is full global data, according to the World Health Organisation's International Agency for Research on Cancer.

While many cases can progress quickly, spreading to other organs and becoming deadly, experts say as many as half of prostate cancers are likely to remain confined to the prostate and are unlikely to become life-threatening.

The problem has always been knowing accurately, and at an early stage, which tumours are most likely to kill.

Although tests for aggressive forms of prostate cancer already exist, experts say they are only moderately accurate.

De Bono said scientists can learn more about prostate cancers by the signs they leave in blood. This allowed his team to develop a test potentially more accurate than those available now and easier for patients than taking a biopsy, he said.

"Our test reads the pattern of genetic activity like a barcode, picking up signs that a patient is likely to have a more aggressive cancer. Doctors should then be able to adjust the treatment they give accordingly," he said in a statement.

GENE ACTIVITY

For his study, De Bono's team scanned all the genes in blood samples from 100 patients in London and Glasgow with prostate cancer. They included some already diagnosed with advanced cancer and some thought to have low-risk, early-stage cancer.

Using statistical modelling, the team divided the patients into four groups according to patterns of gene activity and, after almost two-and-a-half years, they found patients in one group had died significantly earlier than those in the others.

They pinpointed nine key active genes shared by all patients in that group, and when they tested another 70 Americans with prostate cancer, they again found these genes identified patients who survived for a shorter time - around 9 months compared to over 21 months for those without the gene pattern.

The second study by researchers in the United States identified a set of six genes linked to a more aggressive form of prostate cancer in a group of 62 patients at the Dana-Farber Cancer Institute in Boston. The signature divided patients into two groups: one with an average survival time of 7.8 months and the other with an average survival of at least 34.9 months.

The British team said their signature included several genes involved in the immune system - suggesting the immune system is suppressed in patients whose cancers spread around the body.

Commenting on the work in The Lancet Oncology, Karina Dalsgaard Sorensen at Denmark's Aarhus University Hospital, who was not involved in either study, said the findings were welcome and significant.

"These results suggest that a few selected genes in blood samples from patients...can significantly improve the prediction of outcomes," she said.